1. Field of the Invention
The present invention relates generally to measurement-while-drilling tools and specifically to logging-while-drilling tools.
2. Description of the Prior Art
In the oil and gas industry, measurement-while-drilling operations have slowly supplanted the more conventional wireline-logging operations, as a way to develop better and more accurate resistivity and other logs of the borehole and surrounding formation. Additionally, measurement-while-drilling tools have been utilized to provide information in real-time which facilitates and improves drilling operations. For example, information relating to the borehole and surrounding formation may be utilized during drilling operations to make more intelligent decisions about the equipment, drilling muds, and additives which are utilized during drilling. Additionally, the measurement-while-drilling systems can be utilized to pin point with greater accuracy the precise location of oil and gas deposits, and are especially useful in obtaining accurate determinations of the absence of presence of oil and gas deposits prior to the invasion of the formation by the drilling mud and drilling additives.
One significant disadvantage with measurement-while-drilling systems is that the electronics necessary for the transmission and reception of electromagnetic energy must be contained entirely within a single subassembly. This stands in sharp contrast with wireline logging systems which contain some of the necessary electronics at a surface location in a logging truck, and which can utilize the wireline to pass digital commands and data between the surface location and the wireline tool. The fact that the electronics necessary for both transmission and reception are contained within a single, usually steel, collar results in a variety of signal processing problems which must be minimized or eliminated in order to ensure the accuracy of logging measurements. In prior art logging devices, it is not uncommon to have significant problems of mutual coupling between the receivers in the logging tool. Mutual coupling occurs when the responsiveness of one receiving antenna is altered due to the presence of another receiving antenna. Mutual coupling distorts the signal generated by a particular receiving antenna, and corrupts the measurement. The closer the spacing between the receiver antennas, the greater the effect of mutual coupling. Closely-spaced receiver antennas also result in undesirable cross-talk between the receiving antennas. Cross-talk can also occur between transmitters and receivers within the logging tool. These types of signal corruption must be guarded against. Additionally, a certain amount of signal conditioning must be performed to minimize mutual coupling.